Stability of Current Collectors Against Corrosion in APC Electrolyte for Rechargeable Mg Battery

Rechargeable magnesium batteries (RMBs) are highly attractive due to their high volumetric capacity, relatively low cost, and enhanced safety. Signi ﬁ cant progress in the development of RMBs was the introduction of nonaqueous electrolyte solutions that enable reversible electrodeposition of Mg metal. These solutions contain a unique mixture of organo − aluminum and chloro − aluminum species. While these solutions are shown to be stable during cathodic polarization, the presence of chlorine anions in the solution can promote the corrosion process during the anodic polarization. Among all the cell components, the cathode current collector is most prone to corrosive processes. In this study, we characterize the corrosion behavior of different metallic current collectors in standard APC (All-Phenyl Complex) electrolyte solutions by following their electrochemical response and surface morphology changes during anodic polarization. In addition, we investigate the in ﬂ uence of carbon coating on the corrosive behavior of Ni metal. The study shows the ef ﬁ cacy of carbon coating on the current collector. We demonstrate that Ni@C current collector possesses higher corrosion pit initiation potential (2.25 V vs Mg/Mg 2 + ) with respect to the uncoated metallic foils. Improving the anodic stability of the current collectors is a crucial step in the development of a practical rechargeable Mg battery.

Rechargeable magnesium batteries (RMBs) are highly attractive due to their high volumetric capacity, relatively low cost, and enhanced safety. Significant progress in the development of RMBs was the introduction of nonaqueous electrolyte solutions that enable reversible electrodeposition of Mg metal. These solutions contain a unique mixture of organo−aluminum and chloro −aluminum species. While these solutions are shown to be stable during cathodic polarization, the presence of chlorine anions in the solution can promote the corrosion process during the anodic polarization. Among all the cell components, the cathode current collector is most prone to corrosive processes. In this study, we characterize the corrosion behavior of different metallic current collectors in standard APC (All-Phenyl Complex) electrolyte solutions by following their electrochemical response and surface morphology changes during anodic polarization. In addition, we investigate the influence of carbon coating on the corrosive behavior of Ni metal. The study shows the efficacy of carbon coating on the current collector. We demonstrate that Ni@C current collector possesses higher corrosion pit initiation potential (2.25 V vs Mg/Mg 2+ ) with respect to the uncoated metallic foils. Improving the anodic stability of the current collectors is a crucial step in the development of a practical rechargeable Mg battery. Supplementary material for this article is available online Today Li-ion batteries (LIB) are considered the battery technology of reference for current and emerging applications such as transport electrification or energy storage from renewable energy sources. However, current LIB technologies are facing substantial challenges in safety, energy density, environmental impact, scarcity of materials, and price. While most of today's R&D is concentrated on LIB-related systems, shifting towards fully non-Li rechargeable batteries may open up effective ways to overcome such challenges.
In this scenario, the use of multivalent metallic ion batteries has attracted large attention in the battery field due to their high energy density and high abundance. Amongst multivalent electropositive metals, the most interesting case is that of magnesium. Compared to other active metals, magnesium fits better with the requirement previously mentioned: Mg has a lower impact on the environment (extraction, disassembly, recyclability) and: (i) Mg has an advantage over Li in terms of safety, abundance, and very high volumetric specific capacity (2080 mAh cm −3 for Li and 3800 mAh cm −3 for Mg); (ii) Mg has advantage on Na and K in terms of safety (higher redox potential and lower reactivity allowing handling in ambient air) and specific gravimetric and volumetric capacity. Furthermore, in contrast to Li metal-based batteries, electroplating of Mg metal has shown to be relatively stable, and with the appropriate selection of electrolyte, the uncontrolled dendritic metal growth can be mitigated. 1,2 To benefit from the high energy density of the Mg battery, the potential difference between the Mg anode and intercalation cathode should be as large as possible. Unfortunately, high oxidation potentials can lead to the decomposition of the electrolyte solution and possible corrosion of the cathode material and its metal current collector 3 In addition, the electrolyte solution needs to enable reversible Mg metal deposition at low cathodic potentials. All phenyl complex (APC) solutions showed to have a large electrochemical (0-3 V) window that covers most of the available Mg cathode materials. Although APC performs well with some cathode materials, the presence of chloride salts in APC can promote the corrosion of the metallic cathode current collector at potentials exceeding 2 V. [4][5][6] The absence of a stable current collector delay the development of high voltage Mg cathodes which is a key element in the realization of the high-energy-density battery system.
Anodic stabilization can be attained by forming a stable passivation layer on top of the metal surface which can protect it from the corrosive electrolyte solution. Nonetheless, this passivation layer is typically only metastable. In addition, applying oxidative potential can cause destabilization of the passivation layer. Passivation layer facing a localized breakdown results in the dissolution of the layer and leads to the formation of corrosion pits and cracks on the metal surface. 7 These reactions are assisted by the presence of chloride ions, which tend to diffuse to the metal surface, interferes with the passivation layer, and de-attach the protective film. According to previous studies, the corrosion trend has varied with the logarithm of bulk chloride concentration. 8,9 In some specific cases, the early stage of passivation film breakdown is associated with the initiation of pits that are grown in micron-scale and re-passivate. However, in most cases, the growth of the corrosion pits accelerates with increasing positive potential window. 10,11 The corrosion pits are autocatalytic, where the corrosion byproducts that are produced near the pit surface create local conditions that enhanced their growth. 7 Although there are several general studies on corrosion pitting of metals in Mg electrolyte solutions, a comparative case studies on corrosion effect for the different metal in standard Mg electrolyte solution is rarely reported. 5,6 These types of studies are highly important for the reason of identifying candidate current collectors for high voltage Mg cathodes. Recently, Carbon coating is shown to reduce the corrosion process of various metals in different environmental conditions. Wall et al. 3 demonstrated the corrosion protection ability of carbon coating on Al metal current collector in (HMDS) 2 Mg-AlCl 3 /tetraglyme electrolyte solutions. However, there is no study to date that elucidates the activity and prominence of carbon coating on metal current collectors z E-mail: Malachi.Noked@biu.ac.il; ayan.chemical2015@gmail.com *Electrochemical Society Member.
in highly corrosive chloride-based electrolyte solutions such as APC, one of the most practical and high voltage electrolytes till now.
In this study, the corrosion behavior of different metals, namely Ni, W, Mo, and stainless steel (standard coin cell part) in APC (All-Phenyl Complex) electrolyte is demonstrated by using electrochemical stability tests followed by surface imaging. In addition to these standard current collectors, we investigate the prominence of carbon coating on metallic Ni foil (Ni@C), which exhibit significantly higher initiation potential of corrosion (2.25 V vs Mg/Mg 2+ ) as compared to the uncoated Ni, W, and Mo metallic foils Our study pave the pathways towards the development of most stable and efficient cathode current collector in practical rechargeable Mg battery.

Experimental
Materials.-Anhydrous AlCl 3 (99.999%), tetrahydrofuran (THF), and phenyl magnesium chloride in 2.0 M THF solution were purchased from Sigma Aldrich. THF was dried inside the glovebox with activated 4 Å molecular sieves for at least 72 h.
Electrochemical characterization.-Linear sweep voltammetry (LSV) and chronoamperometry (CA) study is conducted by taking different metals (Ni, W, Mo, Stainless steel) and carbon-coated Nickel foil (Ni@C) as current collectors in a three-electrode flooded cell. The Ni and Ni@C metal foils are procured commercially from Gelon Co®. The carbon is coated on both sides of the Ni foil with a thickness of 1 μm. Mg metal foil procured from Gelon Co.® was taken for both counter and the reference electrode to complete the cell structure. The standard synthesis protocol was followed to prepare APC electrolyte. 10 In a typical synthesis process, 0.5 M PhMgCl was mixed with 25 ml of THF in a volumetric flask and stirred for 10 min. After that, 0.25 M AlCl 3 was added to the mixture very gently to balance the exothermic reaction and kept overnight at stirring conditions. After complete reaction, the solution turns to a transparent golden-brown which was used here for all electrochemical measurements. Before electrochemical measurements, all the current collectors were cleaned by an ultrasonic cleaner in isopropanol and acetone solution repeatedly and the native oxide layer from all Mg foils was removed with glass slides and washed with ethanol and dry tetrahydrofuran (THF) before the experiment.
The cell assembly and the electrochemical test were done inside an Ar-filled glovebox. Both electrochemical (LSV and CA) response was recorded in Biologic VMP3 potentiostat at the scan rate of 0.05 mV s −1 . The CA measurement was continued to 48 h at the potential of 1.8 V vs Mg/Mg 2+ . After the electrochemical test, the current collectors were cleaned by THF repeatedly to remove the electrolyte residues and examined under an electron microscope. The scanning electron microscopy (SEM) image and energy dispersive spectroscopy (EDS) was recorded on environmental SEM (Quanta 2000, from FEI). The error margin of SEM-EDS was +/−2%.

Results and Discussion
The anodic corrosion behavior of the different current collectors (Ni, Ni@C, Mo, W, and SS) in APC electrolyte is examined through LSV measurements between 1.5 and 2.45 V vs Mg/Mg 2+ at the scan rate of 0.05 mV s −1 . According to Fig. 1, Ni@C current collector exhibits the highest onset potential (∼2.25 V vs Mg/Mg 2+ ) which suggests the Carbon is an effective passivation layer that is stable up to this oxidation potential. In the case of Ni, Mo, SS, and W current collectors the protective passivation layer is formed by the native oxide layer. The C label of Ni@C refers to the carbon coating on the protected Ni current collector. The surface corrosion is driven by the hardness and thickness of the passivation layer. Ni has an onset potential of ∼2.18 (V vs Mg/Mg2+) and W current collectors exhibit a little lower onset (∼2.0 V) with a sharp increase of current density which reflects the early breakage of the passivation layer. Mo has the lowest onset voltage (∼1.9 V), pointing towards early breakage of the passivation layer. The Mo onset voltage is quite early, and the measured current density is lower than the other current collectors considered in the present study. Although SS, constituting the commonly used standard testing coin cell part, the early onset (∼2.05 V vs Mg/Mg 2+ ) with the rapid increase of current density is reflected from the anodic current profile which further suggests less durability of the protective passivation layer against the corrosion attack and restricting its utilization beyond 2.05 V in APC electrolyte solution. Despite the common use of SS in lithium-ion batteries, it is severely corroded in chloride-containing electrolytes, which limits its utilization in APC 12 We can assign the anodic current response to corrosion of the current collectors and not to the oxidation of the electrolyte solution, as APC synthesized in our lab is stable up to 3 V vs Mg/Mg 2+ . 13 In the polarization curve, beyond the onset potential, the current density begins to rise suddenly indicating the breakdown of the passivation layer and initiation of pits occurred on the surface. Once a pit is formed, the pitting growth is supposed to proceed through active dissolution mode results in the increase of anodic current density. The initiation of pits could be attributed to the adsorption of chloride ions at the interface of the passivation layer and electrolyte solution under the influence of the applied potential at the interface. The soluble complexes readily separated from the passivated layers and move towards the solution and initiating the localized dissolution of the passivation layer and instigating the pits nucleation. The dissolution of the surface protecting film depends on the thickness of the passivation layer, where thin and unstable layers favor the adsorption of corrosive chloride ions. Hence the lower onset potential reflects the thin and unstable surface protective layers against the corrosion, whereas the current density determines the stability and growth of the corrosion pits with increasing applied potential. 14 The sharp increase in the current density indicates the breakage of the passivation layer and the initiation of the uncontrolled corrosion process. 7 The onset potential, where the current density increases significantly, determines the initiation stage for the corrosion process. Moreover, the profile of the Mo current collector consists of two peaks, where the other current collectors exhibit a single sharp increase of the anodic current. The multistep anodic process in Mo is ascribed to the metastable corrosion pitting. 15 In this type of corrosion mechanism, pits are formed at potential well below the pit onset potential but cannot grow with increasing applied potential. The stability of these pits is stable for short duration of time before re-passivation. The construction and deconstruction of pits can result in a polarization curve with multiple peaks as can be observed in Fig. 1. Hence, we found that anodic stability follows Ni@C > Ni > SS > W/Mo. The improved performance in Ni@C is ascribed to the fact that the carbon coating in Ni provides better electrical conductivity, lower internal resistance, and strong mechanical strength as compared to bare Ni foils. The observed results point toward possible utilization of relevant Mg ion cathode, such as chevrel phase (Mo 6 S 8 ) 16 and V 2 O 5 17 in rechargeable magnesium battery, whose practical feasibility was restricted due to suitable current collector. Further, the observation guiding the operating potential of cell assembly in coincell (CR2032) and limiting the working potential to 2.05 V (vs Mg/Mg 2+ ), which is within the operating potential of Mo 6 S 8 . 16 To further investigate the corrosion behavior and the surface morphology of the different current collectors, post-mortem images were taken by scanning electron microscopy (SEM). All the current collectors underwent some corrosion during the anodic polarization up to 2.45 V (vs Mg/Mg 2+ ), as can be seen from the pitting and cracking on the surface of the current collectors (Fig. 2) as compared to the uncycled electrodes. Fig S1a-d (available online at stacks.iop. org/JES/168/080526/mmedia) shows the uncycled Ni, Ni@C, W, and Mo current collector surface respectively. The uncycled electrodes exhibit pinhole and crack-free surface morphology. After the dissolution process, the most dramatic change in the metal morphology was observed in the W current collector, where ∼10-30 μm microcracks are covering the surface (Fig. 2c). The cracking corresponds well to the low set-off corrosion voltage of the W current collector (Fig. 1). The surface of Mo exhibits a similar crack initiation process, shown in Fig. 2d; however, it seems that in the case of Mo, the prorogation of the cracks was less severe than that of W. The reduced corrosion can be explained by the metastable corrosion pitting mechanism, also supported through the LSV measurement. In this type of mechanism, metastable passivation of the Mo surface can protect it from more severe micro corrosion. The multiple peaks during the anodic voltage scan of the Mo current collectors are good indicators for this type of corrosion mechanism. Further, it is observed that in addition to micro cracking, the Ni current collector also possesses fewer pitting corrosion morphologies (Fig. 2a). The surface of the Ni current collector exhibits micro-cracking along with the surface contamination caused by the corrosive chlorides which cover the Ni surface (as also observed from EDS). Although the surface of Ni@C exhibit larger microcracking features (Fig. 2b), than Ni but with more homogeneous and stable corrosion resistance at a given corrosion condition. The diminished changes in the Ni@C surface morphology after polarization and the higher onset corrosion voltage imply that the carbon coating inhibits severe corrosion on the metallic Ni surface and indicates the protectiveness of C coating against surface contamination caused by corrosion.
One of the key facilitators for corrosion in the APC electrolyte solution system is the chloride ions which promote solvation and destruction of possible passivation layers such as metal oxides. To evaluate the presence of Cl species, we conducted EDS measurements on the surface of the current collector (Fig. 3). For the Nibased samples, the surface of the pure Ni has more Al and Cl species than that of the coated Ni@C current collector, observed in Figs. 3a and 3b respectively, which arises due to corrosion and potentially due to the tendency of more AlCl3 to adsorb on the corroded' chloride-containing surface. Further in Ni@C, the carbon coating protects the surface from Cl penetration which is reflected by the higher amount of available Ni on the surface, which indicates that the carbon coating decreases the corrosion process significantly. The highest relative percentage of Cl species was observed for the W surface. The enhanced migration of Cl ions towards the W surface can cause rapture in the natural W oxide layer, which leads to the observed cracking of the metal surface (Fig. 3c). 8 The lowest percentage of Cl species was observed on top of the Mo surface. This can explain the reduced surface corrosion that was observed for Mo (Fig. 3d).
To compare the stability of the metal current collectors above the operating potential of suitable cathodes (Mo 6 S 8 or V 2 O 5 ) in APC electrolyte solution for rechargeable magnesium battery, we conducted chronoamperometry (CA) measurement at a constant potential of 1.8 V vs Mg/Mg 2+ and follow the current changes for 48 h for Ni, Ni@C, W and Mo, as displayed in Figs. 4a-4d respectively. It is observed that all the current collectors exhibit a current profile with negligible current densities and stays stable for 48 h. The respective insets showing the initial current profile for short duration of time. For Ni, and W current collector the sudden drop of initial current density indicating the formation of passivation layer as shown in the inset of Figs. 4a and 4c. A slightly different stabilization behavior is observed for the Mo current collector (inset of Fig. 4d). Interestingly, the initial current decay presents small fluctuation throughout the stabilization process. This type of current behavior can be explained by the formation of metastable corrosion pits as well. The current fluctuation can correspond to the formation and destruction of a metastable passivation film during anodic polarization. For Ni@C current collector, the lowest value of current density is observed at the initial stage as well as after stabilization, signifying the growth of a stable passivation layer (Fig. 4b). The SEM images of the current collectors after the CA measurement for Ni, Ni@C, W, and Mo are presented in Figs. 5a-5d respectively. The images demonstrate that even at low anodic potentials and relatively low current density, the Ni and W metals exhibit fewer corrosion pitting on the surface (Figs. 5a and 5c). Some rearrangement of the Mo surface is observed due to the metastable corrosion pitting mechanism, after the CA procedure as shown in Fig. 5d. Our long CA experiment implies that although these metals are suitable as a current collector for short-term measurements, they are not suitable for long-term measurements even with a cutoff voltage of 1.8 V in APC electrolyte solution. On the other hand, the SEM images of the Ni@C current collectors demonstrate that no significant morphological change occurred during the long constant potential step (Fig. 5b), confirming excellent corrosion resistivity. The presence of small pitting in pure Ni current collectors and their mitigation after carbon coating in the Ni@C is another evidence for the superior anodic stabilization achieved by the carbon coating.

Conclusions
The surface corrosion effect of Ni, Mo, W, SS, and carbon-coated Ni (Ni@C) metals in APC electrolytes was investigated by electrochemical methods and electron microscopy imaging. The onset potentials recorded by linear scan voltammetry enabled the formulation of a corrosive stability trend of the metals as follows: Ni@C > Ni > SS > W/Mo In addition, we found that Ni@C presents a higher corrosion pit initiation potential (2.25 V vs Mg/Mg 2+ ) compared to the uncoated Ni metal. The lack of changes in the surface morphology of the Ni@C surface during long anodic polarization at 1.8 V demonstrates the longterm stability of the coating at relatively high potentials. In summary, carbon coating is an effective approach to improve the surfaces corrosion stability of abundant metal current collectors in chloriderich electrolyte solutions. Our study shed light on the corrosion resistive property of carbon that explores the possibility of using alternate carbon-based current collectors like bucky paper, graphene sheets even with reduced thickness. These solutions might offer improved energy density of the system through an economically profitable approach. Henceforth, further effort in the development of cost-effective methods to produce practical cell components is required before rechargeable Mg battery technology is fully realized.